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I have been using this from 2010 - 3M Privacy Filters:
Helps to keep confidential information private,
Data on screen is visible only to persons directly in front of the monitor, passersby see only a dark screen similar to shutdown system. This is not password protected or technically protected, just an illusion.
Dont consider this as an software protection.
This is just to make sure passerby dont see whats on your monitor window open.
Observed it Improves screen contrast and reduces screen glare and reflections.
it was $5 when i bought and now its too high at #38's !!
There is something similar that I've seen for license plates. The cover basically has vertical "lines" that distort the image when viewed from the side, the idea being that "red light" cameras would have not be able to photograph the plate. Whether it worked for its stated purpose, I don't know, but without a doubt it blocked my view of the object behind the cover when viewed from any significant angle. The image remain clear from straight on, with some vertical stripes "ghosted" through the cover. Obviously this is different from the 3M screen covers, but it is the same basic idea, so I would expect the 3M cover to work quite well at stopping "peekers". I can't speak to durability, but judging from the age of some of the 3M post-it notes around my desk, I would expect it to outlast cockroaches and be Chuck Norris tough.